There is a need for an electrically small, low profile, band switchable VHF antenna which can be mounted on a metallic surface such as a truck or railway car rooftop. Such antennas could be used to communicate with earth satellites, and therefore preferably should be polarized such that the plane containing the electric field is horizontal in normal use. It would also be useful to have the antennas design such that it could receive circularly polarized signals.
Antennas which are used today typically have elements of the order of 1/4 wavelength in length, such as whip antennas. However, such antennas are not low profile, as they must be mounted with their axes perpendicular to a conductive ground plane. Dipoles have balanced elements each 1/4 wavelength long, but cannot be low profile above and in close proximity to a conductive plane.
The small loop antenna is a well known structure, and many forms have been devised as shown in FIGS. 1A, 1B and 1C. For example, in FIG. 1A the equivalent circuit of a multi-turn loop antenna 1 is shown. An incident electromagnetic (EM) wave induces a small voltage which is shown as equivalent to a voltage generator 3 in series with the loop 1. Small equivalent series resistances 5 and 7 represent radiated energy from the antenna and ohmic losses due to parasitic resistances respectively. The loop itself exhibits inductance. At VHF frequencies and below, ferromagnetic materials may be included in the core of the loop (not shown) to increase the radiation resistance (and hence the efficiency) of the antenna.
In general, the impedance of the antenna at the loop terminals is undesirably highly reactive resulting from the series of a very small resistance value and a high value inductance. The inductive loop can be resonated with a capacitor 9, but this results in an equally undesirable very high output impedance.
The impedance can be transformed to a lower value by the use of a capacitive tap configuration as shown in FIG. 1B, wherein the capacitor 9 is split into two series capacitors 9A and 9B, with the output signal being taken across one of the capacitors. Alternatively, the impedance can be transformed to a lower value by use of an inductively coupled loop 11, as shown in FIG. 1C.
Antenna performance is characterized by antenna efficiency Ea, which is given by EQU Ea=10*log(Rr/(Rr+Ro))
Where Rr is the useful radiation resistance and Ro is the equivalent resistance representing the sum of all ohmic losses.
Electrically small loops have very small values of radiation resistance. For example, a loop of approximately 10 square inches in areEt (e.g. as may be formed by a 1.75" radius circle, or by a rectangular shape of equal area) has a radiation resistance of approximately 0.08 ohms in free space at 150 MHz. However, such a loop also has a considerable parasitic resistance arising from the well known skin effect and the equivalent series resistance (ESR) of capacitors used to transform the output impedance.
The skin effect limits the flow of radio frequency (RF) currents to a surface Layer in a conductor. For example, in a copper conductor at 150 MHz, the RF current is diminished to 1/e of its surface value in 5.4 microns, resulting in a resistance of 3.2 mohms per square of conducting surface. As a result, large diameter wire or wide metal strip conductors are necessary to minimize ohmic losses. For example, as shown in FIG. 2, a circular cross section antenna 13 fabricated from a 0.25 inch wide copper strip, 2.5 inches in diameter has a metallic resistance of 0.08 ohms, which is the same as its radiation resistance.
Capacitor parasitic series resistances amount to approximately 80 mohms per capacitor (which can be mitigated by use of multiple capacitors in parallel).
Such a loop antenna with two tuning capacitors in parallel (ESR =0.04) thus has total parasitic resistance of 0.12 ohms and a radiation resistance of 0.08 ohms, and thus an efficiency of -4dB.
The inductance of such a loop is approximately 80 .mu.H, so that the loaded Q of the antenna is 190 at 150 MHz, resulting in a 3 dB bandwidth at 0.79 MHz. This is very narrow, but the useable bandwidth is considerably smaller yet, because in fact only a 1 dB degradation can be typically accommodated.
As is known, a repelling force is exerted on parallel flowing currents. The effect of this is to cause the current flowing in the circular cross section strip shown in FIG. 2 to tend to flow at the edges of the strip and to be attenuated in the center. If the current were uniform, the highest magnetic flux density would occur in the center of the strip. Currents flowing in this strip would require greater energy than at the edges, and consequently the current becomes non-uniform, thereby defeating the lower resistance strategy.
Also, the wide spatial distribution of the current results in diminished mutual coupling between parallel current components in the same conductor, which results in a lower inductance. The far field generated by currents in a loop is the superposition of the contributions of the incremental currents within it. If the loop current is concentrated toward the edges of the strip as a result of strip width, the center portion will not contribute significantly, and the antenna will behave as two loosely coupled loops, switch concomitantly poor performance.
The radiation resistance of a multi-turn antenna increases as the square of n, the number of turns. Since the skin effect losses increase proportionally to the wire length used in the antenna, the efficiency of a multi-turn loop antenna is improved over a single turn loop antenna.
However, the inductance of the loop antenna also increases as the square of the number of turns, and at 150 MHz, the requirement for a large antenna coil area results in impracticably high inductance values. For example, two fully coupled turns of the dimensions above would have an inductance value of 320 nH, and require a very small value tuning capacitor. This becomes impractical for many reasons, including susceptibility to de-tuning by environmental stray capacitance.
It may be seen that conventional loop antennas provide a very narrow band response and in general are inefficient.